Conjugated conducting polymers have generated great interest because of their moderate charge mobilities, ability to be redox doped to highly conducting compositions, along with the ability to change optical properties reversibly. Conducting polymers show potential in commercial applications including color changing materials, conductors, antistatic coatings, electronic components, photovoltaic devices and light emitting devices. Commercial products from conducting polymers are potentially cost effective, more easily processed, lighter in weight, and more mechanically flexible, than products fabricated from alternate material in existing technologies. A class of conducting polymers, polyheterocyclics, which include polypyrroles, and polyfurans, are a well-known class of conducting polymers. Such polyheterocyclic conducting polymers have been extensively studied in electrochromic devices, photovoltaic devices, transparent conductors, antistatic coatings, and as the hole-transport layer in light emitting diodes. Appending a 3,4-alkylenedioxy bridge on the heterocycle allows a modified polyheterocycle, where the bridge does not cause an undesirable conformational change in the backbone of the polymer, and the electron donating effect of the oxygen substituents increases the HOMO of the conjugated polymer, reducing both its oxidation potential and its electronic band gap.
The poly(3,4-alkylenedioxypyrrole)s are conjugated polymers that show great potential due to their wide range of band gaps, low oxidation potentials, biological compatibility, and flexibility towards functionalization. However, their progress has been limited to some degree because of difficult and inefficient synthetic pathways to 2,5-dihydro-3,4-alkylenedixypyrrole monomers. The poly(3,4-alkylenedioxyfuran)s are not well known, having only been reported in a Japanese patent application.
The pyrrole and furan-based 3,4-alkylenedioxyheterocycle polymers are commonly prepared by the chemical or electrochemical oxidative polymerization of 3,4-alkylenedioxypyrroles or 3,4-alkylenedioxyfurans. However these monomers, with hydrogen in the 2- and 5-positions of the heterocycle ring, are not readily synthesized in an efficient manner. These syntheses generally involve multiple steps where one or more of the steps are difficult or poor in yield. The chemical polymerization of 3,4-alkylenedioxypyrroles is typically carried out with an oxidizing agent, such as ferric chloride or cupric chloride. This results in a doped polymer which is often insoluble unless a polyelectrolyte, such as poly(styrene sulfonic acid) is used to yield a processable polymer solution. In many polymers, the use of chemical oxidants such as ferric chloride results in a material with trapped metals, which are difficult to remove. Furans are sensitive to and undergo decomposition in their presence of acid. The poly(styrene sulfonic acid) complexes of the polymers have excess acid sites present in the final material that can degrade materials that are commonly used as electrodes.
Poly(3,4-alkylenedioxythiophene)s are often prepared in a similar manner to that of the poly(3,4-alkylenedioxypyrrole)s, where again a doped polymer results upon polymerization. To obtain the neutral polymer, a Ni(0) complex promoted polycondensation of 2,5-dihalo-3,4-ethylenedioxythiophenes has been employed. However in this manner unprocessable polymers (Yamamoto et al., Synth. Met. 1999 100, 237; Yamamoto et al., Polymer 2002, 43, 711) result or only low molecular weight materials are formed (Tran-Van et al., Synth. Met. 2001, 119, 381; Tran-Van et al., J. Mater. Chem., 2001, 11, 1378).
Balk et al. U.S. Pat. No. 7,034,104 discloses that 2,5-dihalo-3,4-dialkyloxy- or 2,5-dihalo-3,4-alkylenedioxythiophene can be polymerized with an acid. Although claimed as comprising an acid catalyst, the polymerization required a stoichiometric equivalent up to a 20 fold excess of acid, relative to the monomer in solution, at temperatures in excess of 100° C. to achieve a doped polymer with conductivities between 19 and 255 S/cm after removal of the solvent.
To achieve a well defined polymer structure of poly 3,4-ethylenedioxythiophene, the solid state polymerization of 2,5-dihalo-3,4-ethylenedioxythiophene was studied by Meng et al. J. Am. Chem. Soc. 2003, 125, 15151. The polymerization of the dibromo monomer occurs at room temperature but only over an extremely long period of time. The polymerization occurs in a significantly shorter period of time in the crystalline state with the dibromo monomer when heated to slightly below the melting point of 96-7° C. but not in the liquid state when rapidly melted. The compound polymerized spontaneously at about 140° C. if slowly heated in a large sample, but this polymerization was attributed to the accumulation of catalytic impurities and no polymerization was possible in solution. The diiodo monomer, melting point 185-8° C., can be polymerized but only at temperatures in excess of 130° C. The dichloro monomer, melting point 60-2° C., does not polymerize in the solid state. Differences in the polymerizability are attributed to the difference in the crystal structure. The distance between intermolecular halogen atoms relative to the sum of their van der Waals radii was correlated with the conclusion that polymerization is promoted when the sum of their van der Waals radii exceeds the distance between intermolecular halogen atoms, as in the dibromo and diiodo crystals but not in the dichloro crystals. Another correlation made with polymerizability was with the carbon-halogen-halogen angle in the crystal structure. This angle is nearly a right angle in the dibromo and diiodo crystals, 106.7° and 101.6° respectively, but the atoms are co-linear, 180° in the dichloro crystal. Hence, this study indicates that polymerization should not be expected in the liquid or solution state, and should only be expected in the crystalline state if the crystal structure is known to have the proper spacing and orientation of monomer units within the crystal.
Although the polymerization of 2,5-dihalo-3,4-alkylenedioxythiophenes was first reported eight years ago, the polymerization of the 2,5-dihalo-3,4-alkylenedioxypyrroles and 2,5-dihalo-3,4-alkylenedioxyfurans has not been reported with or without a catalyst. It would be desirable to prepare poly(3,4-alkylenedioxypyrrole)s or poly(3,4-alkylenedioxyfuran)s by the polymerization of these dihalo monomers as they can be prepared in much higher yield than the 3,4-alkylenedioxypyrroles and 3,4-alkylenediloxyfurans and are much more easily purified. This should lead to a reduction in the cost of the polymers. Some of the applications for polymers prepared by such a method could be for electrochromic windows, mirrors and displays; field effect transistors, supercapacitors, batteries and other electronic components; electronic paper; camouflage; anti-stat conductors; and photovoltaic devices.